1. Field of the Invention
This invention relates to improvements of an optical image reader for use in a facsimile system, an optical character recognition (OCR) system, a copier and the like.
2. Description of the Related Art
An optical image reader has been developed for use in a facsimile system, an OCR system, a copier and the like.
U.S. Pat. No. 4,045,817 discloses such a conventional optical image reader operating in a charge storage mode. This conventional optical image reader comprises a plurality of photosensing elements PD.sub.1, . . . , PD.sub.n, a plurality of MOS transistor switches (MOS switches) M.sub.1, . . . M.sub.n, a plurality of MOS capacitors MC.sub.1, load resistors RL.sub.1 and RL.sub.2, a power source and a scanning pulse generator. In this optical image reader, each photosensing element is connected to a source electrode of the corresponding MOS switch. The signal line is commonly connected to the drain electrodes of the MOS switches. A gate electrode of each MOS capacitor is connected to a gate electrode of the corresponding MOS switch and a corresponding output terminal of the scanning pulse generator. Each drain electrode of MOS capacitors is commonly connected to the noise line. These signal and noise lines are respectively connected to the input terminals of the differential amplifier, and further respectively connected to the power source through load resistors RL.sub.1 and R.sub.2.
By applying a scanning pulse to the gate electrode of each MOS switch sequentially from the scanning pulse generator, each MOS switch is turned on sequentially. Then, a photo-signal from the photosensing element and a spike noise from the MOS switch appear on the signal line. At the same time, a spike noise from the MOS capacitor appears on the noise line. These outputs which appear on the signal and noise lines are differentially amplified by the differential amplifier. The MOS switch used in this conventional image reader has a P channel MOS structure, or an N channel MOS structure. FIGS. 1(A) and 1(B) show the ON-resistance characteristics of the P channel MOS switch and the N channel MOS switch respectively. As is apparent from FIGS. 1(A) and 1(B), the ON-resistance of the MOS switch becomes high as its input voltage approaches a gate voltage which is V.sub.DD in the N channel MOS switch or V.sub.SS in the P channel MOS switch. This phenomenon has a bad influence on the switching of the MOS switch. Thus, the voltage range of the MOS switch must be limited to a certain range which is free from the influence.